With this new version of Mint/Ubuntu the AMD proprietary fglrx-driver with Catalyst Control Center is not supported and installable anymore and the dGPU is too old for the new amdgpu-driver. So my system is running on the radeon-driver now.

With the fglrx-driver and Catalyst Control Center i was able to activate and deactivate the AMD dGPU, depending on my use-cases, manually.
With the radeon-driver now it's a bit different.

When i check my current GPU state with

$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch

the result is always

0:IGD:+:Pwr:0000:00:02.0
1:DIS: :DynOff:0000:01:00.0

This means, that the iGPU is active and the dGPU is deactivated (by software/driver).
Since i have (like all modern laptops) a muxless device, the entry 0:IGD looks good, but 1:DIS should change its state, when needed, to something like this

that i can force to use the AMD dGPU with a specific application (i.e. glxgears) like this

$ DRI_PRIME=1 glxgears

but not as the default GPU, used by the whole system (like with the Catalyst Control Center). Is this even possible (yet) with the open-source radeon-driver?
And is there a way to use something like auto-GPU-switching (like, when the Intel iGPU has reached maximum load, the AMD dGPU will automatically be switched on) ?